Huge space rocks crash into our planet on a regular basis, and terrestrial impact craters are the evidence. Find out where you can see these craters, and why we don't see even more of them.

Definition

An impact crater is a landform left behind by the collision of a meteoroid, asteroid, or comet with a planet or moon. The classic crater is a bowl-shaped depression with a raised rim, but impact craters that have been eroded or filled over time can also be recognized by patterns of fault lines, mineral deposits, or through geophysical techniques, such as the mapping of gravitational or magnetic fields. Terrestrial impact craters are those found on Earth.

Barringer Crater, Arizona

Many people, even those who have seen the famous Barringer Crater in Arizona or learned how an asteroid strike probably doomed the dinosaurs, might be surprised to learn that the Earth has been hit often -- in fact, more often than the Moon or Mars. Why don't we see huge craters everywhere? Sometimes the answer is that we're living right inside one! For example, Middleboro, Kentucky, Nördlingen, Germany, and the lower Delmarva Peninsula in Maryland all lie within impact craters.

Boundaries of the Chesapeake Bay impact crater

Where Did They Go?

Most impact craters on Earth get disguised by the same forces that make this a living planet. Erosion wears down the crater's sides. Infilling happens when sediments, ice, windblown dust or other substances fill the depression and smooth out its contours. Volcanic activity can bury craters under layers of lava, and can also act as a decoy -- many impact craters look a lot like the craters of volcanoes, and that's what they were assumed to be until recent research proved otherwise. Tectonic forces, the movements of huge plates of Earth's crust, guarantee that ancient rocks that might hold impact evidence get subducted and 'recycled' on a regular basis. Finally, the fact that over 70 percent of Earth's surface is covered by oceans means that there's plenty of room for space rocks to hide under water.

Identifying an Impact Crater

Even with all these geologic forces at work, at least 170 terrestrial impact craters are recognized today, and geologists are still investigating many other suspicious-looking sites. The simplest craters are flat-bottomed bowls, surrounded by a single rim of ledges or cliffs, but depending on the size of the impactor and the underlying material, a complex crater can contain internal structures such as hills or plateaus. These are formed by the rebounding of the Earth's surface following an impact. Craters may be ringed by a series of ridges, like ripples in a pond, or by 'rays' of material cast out in all directions. Even when no evidence remains above the surface, a strike leaves its signature. Telltale cracks and faults can be detected by sensing devices, and impact materials -- diaplectic glass and other substances that can only be formed during a violent collision -- remain beneath the crater floor, even where no trace of the original asteroid survived the hit.

The tiny Kaali craters in Estonia were once thought to be the result of sinkholes or volcanic activity

Crater Locations

Impact craters have been found on every continent, and also on the ocean floor. The best-preserved examples tend to be on continents with stable cratons, which are ancient, quiet, thick sections of crust in the centers of tectonic plates, far from the geologically-active plate boundaries. Some of the oldest cratons are in northern Canada, Western Australia, and the Baltic region of northern Europe, but impact craters have been found in other places, too. The map below shows the distribution of many of Earth's craters.

Summary

Although terrestrial impact craters can often be hard to spot, they are not uncommon. These landforms, which bear witness to repeated meteoroid strikes on the Earth over the years, are usually disguised by the same natural processes of wear and tear that affect all geological structures on a living planet. Whether simple or complex, large or small, impact craters share common features that are important clues to their extraterrestrial origin. There may be one closer to you than you think!

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